As is described in Patent Document 1 and Patent Document 2, glass, plastic, a single crystal of lithium niobate (LiNbO3) and or the like, has been previously used as a material for an optical component, e.g., a lens, mounted on an optical device, e.g., an optical pickup.
The glass and the plastic have high light transmittances and are easily worked into desired shapes. Therefore, glass and the plastic are primarily used for optical components, e.g., lenses. On the other hand, the single crystal of LiNbO3 or the like is primarily used for an optical component, e.g., an optical waveguide, taking advantage of electrooptic properties and birefringence.
Further miniaturization and thickness reduction have been required of optical devices, e.g., an optical pickup, including such an optical component.
However, refractive indices of known glass and plastic materials are less than 1.9 and, therefore, there are limits to miniaturization and thickness reduction of the optical components and the optical devices including the known glasses and plastics. In particular, the plastic has a disadvantage that the moisture resistance is poor. In addition, there is also a disadvantage that it is difficult to effectively transmit or condense incident light because birefringence may occur.
On the other hand, the single crystal of LiNbO3 or the like has, for example, a relatively high refractive index of 2.3. However, the single crystal of LiNbO3 or the like has a disadvantage that it is difficult to use for optical components, e.g., a lens, because birefringence occurs and, thereby, the use is limited.
Examples of materials which cause no birefringence and which can provide excellent optical properties include translucent ceramic containing Ba(Mg,Ta)O3 based perovskite or Ba(Zn,Ta)O3 based perovskite as a primary component. These are described in, for example, Patent Document 3 and Patent Document 4, respectively.
However, such translucent ceramic containing Ba(Mg,Ta)O3 based perovskite or Ba(Zn,Ta)O3 based perovskite as a primary component is a polycrystalline substance. Therefore, there is an essential problem in that voids are liable to remain in a sintered body thereof. That is, light is liable to scatter when light passes through the translucent ceramic if many voids are present in the sintered body, and as a result, the light transmittance decreases.
The optical properties, e.g., the refractive index and the Abbe number, of the translucent ceramic containing Ba(Mg,Ta)O3 based perovskite as a primary component can be changed by allowing Sn and/or Zr, which are tetravalent elements, to substitute for a part of Mg and/or Ta. The amount of change increases as the amount of substitution increases. However, the upper limit of the amount of substitution is a low 0.40 and, therefore, it is difficult to change the refractive index and the Abbe number to a large extent. For example, a change merely within the range of 2.071 to 2.082 can be obtained in terms of the refractive index.
Likewise, it is difficult to change the refractive index and the Abbe number of a translucent ceramic containing Ba(Zn,Ta)O3 based perovskite as a primary component to a large extent.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-127078 (all pages, FIG. 1)
Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-244865 (claim 6, a paragraph [0024])
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-75512 (all pages, all drawings)
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2004-75516 (all pages, all drawings)